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Ring Plates.

mistaking the one for the other. The rivet holes, one in six, for
the back long edge must be set out so as to bring the joints
F and B (Fig. 311) into exact relation with each other. B B are
therefore marked upon the Front Plate, and two methods occur
by which the intermediate holes may be traced: one involving
the use of the thin strips, and the other being the placing of one
plate upon the other, on blocks as at 20. The latter method
seems preferable, because all the holes may be marked on the
back edge of Front Plate, one in six drilled, and then traced
through to the Back Plate, VIII. The manhole is next marked
off, with-its rivet holes, but is not cut out till in position. The
butt strap is prepared by planing; heating and pressing to correct
curves between dies; then marking off all holes, but drilling only
three on each edge. It is next applied to the plate, these holes
marked through and drilled.

VIII. Back Ring Plates.—These are also in three, and of
equal length. They are marked as in the last example, and if
care be taken, the horizontal joints of the plates II. and V. will
be in line with each other. This is a necessity, so it is advisable
to keep the vertical centre line of the boiler well in view, on all
these plates I. to VIIL, during the whole of the marking off.

We may now bolt together the whole of the shell plates
through such rivet holes as have been drilled, and place the
boiler upon the cradle A A, Figs. 293-4, Plate XIII. The drill
spindle is adjusted as there described, and all the holes in the
ring plates drilled right through. There are two principal forms
of rivet holes required, as shewn at 37 and 38, the former being
for machine and the latter for hand-riveting. In 37 the arridge
is just taken off, while 38 requires 'a deep countersink, but both
may be given by the tools 21 (a and b). 2ia is applied from the
outside, and withdrawn when the hole Is finished. 2 lA is then
passed through from the inside of the boiler, and fastened in a
special slot as shewn. Its teeth cut left-handed, so the machine
need not be reversed, but the backward feed is given by hand,
and the depth gauged by a mark on the drilL All the shell rivets
are like 37, excepting those in the back flange, and even they
-y be machine-riveted, as will be shewn. The manhole is taken
: by drilling holes round its circumference close together, then